The new Dreepy evolutionary line, to me, brings up some pretty interesting questions about the Pokemon world’s evolutionary biology. They’re apparently aquatic Pokemon that lived in the ancient past that became spirits once they became extinct, and now fittingly have the “ghost/dragon” typing in the modern day. What’s interesting about this is you must wonder, why isn’t this more common, that being why isn’t there a ghost type variant of every existing Pokemon, since all Pokemon are living creatures that shed their mortal coil eventually all the same. This seems to imply that becoming a distinct ghost-type pokemon is something only some are capable of, while the rest just become normal wraiths like Pikachu did in the Pokemon Tower episode and presumably eventually pass on. Might there be some “metaphysical” (or possibly just physical, since this is just how things work in this world) laws that determine how adaptive a Pokemon’s spirit is? And judging what we know of Pokemon that are suspected to have once been the departed spirits of humans (such as Yamask and Gengar) and how different they look compared to humans, how many ghost type Pokemon might be the result of the ghost of a known or unknown species of Pokemon? Hell, if we could somehow find the spiritual version of “genes” would it be possible to trace common ancestry with their mortal relatives, and add ghostly branches to the Pokemon tree of life? The implications of this are both overwhelming and exciting.
Yeah, it’s an interesting problem. I really love the idea of a Pokémon that’s not a fossilised prehistoric creature, like so many we’ve seen before, and instead the ghost of an extinct creature, but it does raise that question – why this Pokémon? Why is Dreepy unique (well, not quite unique; Galarian Corsola seems similar, but they’re obviously unusual)? I would not actually default to thinking that Dreepy became lingering spirits because of something inherent to them, though. In folklore, people become ghosts because of something about the way they died – maybe they have “unfinished business,” or weren’t given the burial rites their culture requires, or were killed by a particular monster, or just died in a particularly unpleasant way that somehow damaged their soul and prevented them from moving on to wherever spirits are supposed to go. In Pokémon, we often aren’t explicitly told where Ghost-types come from, but when we are, my impression is that it’s more often a magical or spiritual cause than a biological or scientific one (of course, then the follow-up question is whether we’re supposed to believe what we’re told about Phantump, Sandygast, etc. or just see it as a mystery yet to be solved). Given what little we know, my first guess would be that Dreepy exist in their current ghostly form due to something about the nature of the event (or competing species, or predator) that drove them extinct. Maybe their species was wiped out by something unusually sudden or traumatic, or maybe there was some Ghost-, Dark- or Psychic-type predator (now extinct itself as well) that could manipulate and damage souls, or maybe – just maybe – they were the victims of some kind of spiritual calamity, like an eruption of the spirit world into the “real” world. That’s the sort of place my mind goes when you raise the question, at any rate. I think in the absence of anything more explicit from an official source, the “correct” answer is probably whatever you feel is the more potentially interesting.
Pokémaniacal 
Something I should add to this: Is that the fact that both dragapult and drakloak seem inclined to care for young dreepy, indicating that these spirits of an extinct Pokemon are reproducing (which was technically canon up to this point as ghost types can breed, but otherwise hasn’t been hard confirmed) and have adapted into some new ecological niche (only pokemon would have ecological niches that are filled primarily by ghosts |D). I suppose that’s what led me to believe that their spirits had some sort of “genetic mutation” equivalent that led them down this way, though that was a poor assumption on my part since it assumes that ghosts adapt the same way as living organisms, which is very much not likely the case. I think it is fun to speculate that they still do pass “something” on as spirits in a sort of rudimentary evolutionary process. Perhaps their makeup is more sensitive to their will and emotional state rather than a genetic code? (worth noting, apparently drakloak become emotionally distraught when they lose the dreepy on their head, which I find incredibly adorable) And perhaps what they pass on to their offspring is an aura of “stability” which helps keeps their personal will and emotion at a frequency that helps them maintain their current form. But of course this is just me speculating about the evolutionary process of fictional ghost animals in a children’s cash grab gaming franchise, so any guess is as good as mine. XD
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Asking questions about Dreepy and galarian Corsola can’t help but becoming asking about all ghost pokémon, because then you have to consider why are those two outliers? These two rather threw a wedge on the way I understood ghost-types, but I cling to my theory: they’re spirits that are born and made out of emotions and spiritual energy, thusly feeding on these as sources of energy. (Messy rant below, I like this theme but don’t have my own blog so I’ll spam here, sorry!This is as much summarizing as I am physically capable of because I’m bad at structuring my thoughts!)
Think about it: Banette are outright said to emerge from and be made of grudges, which their pre-evolved form, Shuppet, congregate around and eat. Mimikyu are inordinately associated with loneliness, low self-esteem, and all that fun stuff. Gastly, Haunter, and Gengar seem extra fond of trickery, although they deal in all types of ghostly deeds. Plenty of ghosts seem to mostly prey on living creatures by draining their vitality, but- especially the smaller ones- also startle and frighten them constantly as… maybe both play and a less risky (for both parties) feeding mechanism?
Oh, sure, there’s Yamask and Phantump, but if anything those guys only leave me more convinced that ghosts, in general, are independent creatures that spring from other’s emotions, because look at a Yamask’s behavior and tell me that’s consistent with a specific deceased human person. They never seek out dead families or unfinished business. If you catch a cute Yamask and announce you’ll name him Sphinx, he’s not going to become enraged at you and shout his name is George, or cry a single tear for his life lost as he resigns himself to this. He’s going to coo a little and ask to be pet. Compare a Yamask to the literal undead humans we see sometimes, in marvelous bridge or old chateau. Look at Shedinja. I’m not quite sure what a mew-damned Shedinja would be incarnating in my hypothesis, but it sure as hell isn’t an undead Nincada.
And if these ghosts are overall ordinary creatures, just magic-based instead of carbon-based, it explains how they can have natural habitats and regional variants- if they’re born when parents scatter spores or something, that bloom with plenty of shade and dreariness, then it handily explains why a sufficiently angsty child with a dollhouse can generate a Shuppet in Hoenn but not Unova: there need to be spores, but those can be dormant and unnoticed, like how fungi in our world seem to pop up out of nowhere.
Well, now that I’ve talked for four paragraphs about how I understand ghosts, let’s see if I can actually fit Dreepy, the subject OP was talking about, in here, huh? Well, what if they never were all the way there to dead? And neither were Corsola? What if they’re a more extreme example of alolan Marowak? To adapt to predation, these creatures became more sensitive to ghostly energies, and less present in the physical world, as such becoming ghosts themselves. If all pokémon come from a common ancestor, that’d explain how ghost pokémon exist at all. Ghosts are sea creatures and these three strains are the cetaceans. The OG ghost ancestor probably looked more like a Ditto or a Grimer or something to originate the body plans we see, of course, but… well, that’s my take.
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Love the idea of ghost-spores being activated by psychological triggers!
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